Electrolytic stripping



U ited States Patent" No Drawing. Applicatiou septemher 21, 1956 Serial No. 611,377

Elaims. 01. 204 146) I 'The invention relates to removing metal coatings from aluminum byelectrolytic stripping.

Theterm aluminum as hereinafter used in the speci-' fication and claims is intended in a generic sense to include commercially pure aluminum and alloys which are predominantly, or contain a substantial amount of, aluminum. I i

There are numerous industrial or engineering applications whieh utilize chromium plated aluminum, The formability and light weight of an aluminum base object provided with a surface coating of chromium, which is hard, wear-resistant and possesses a low coeflicient of friction, furnishes a most desirable combination and relationship of metals. With usage, however, the chromium surfacecoating may become worn in certain areas, whereupon it is desirable to remove the chromiumv deposit and replate the aluminum base object. There are certain applications where it is essentialthat the original dimensions of the underlying aluminum object, be retained. Also, it is sometimes necessary to remove or strip improperly applied metal deposits or coatings from an aluminum object. Such deposits may be ofnickel, copper, zinc or other metals, asfwell as chromium] Prior art methods for strippingchromium and other metals from aluminum have been painstaking, costly and difficult to control, particularlyv from the standpoint'of selectively removing the surface coating or overlying coatings without removing some metal from. the aluminum base object. A primary object of the invention is to provide an improved process for stripping a metal coating or coatings from aluminum in a manner which does not result in the removal, minum. Another object of the invention is to provide aprocess of stripping metal coatings from aluminum in a manner which eliminates the necessity for careful attention or close control of the process. T he process isv self-reg ulating in the sense that whenthe overlying metal coating or coatings are removed from the aluminum, the action of stripping ceases, leaving the aluminum object clean, without etching, dissolving or removing any of the aluminum.

Still a further object of the invention is to remove various metal coatings from aluminum in a rapid manner, while accomplishing the foregoing objects. I,

These, and other objectsand advantages of the invention will be apparent from the following detailed description. i

In accordance with a preferred embodiment of the invention, a stripping bath is provided which consists essentially of a dilute aqueous solution of sulphuric acid having a maximum concentration of approximately by volume, or a specific gravity of 1.20. A solution as dilute as 2% by volume may be used. A 5% solution, or a solution having a specific gravity of 1.06, is preferred. A metal coated or plated aluminum object is made the or loss in weight, of aluanode in the dilute'sulphuric acid stripping bath, the

cathodes being of carbon or lead.

The metal coatings to be removed or stripped from the aluminum may be deposits of chromium, nickel, zinc or copper, or a composite coating comprising a plurality of layers of such metals. In fact, the process of my invention is suitable for removing from aluminum any metalwhich is soluble anodically, in a, dilute sulphuric acidusolution. The coating metals may have been applied to the aluminum object in accordance with any of the procedures known'in' the art, whether by direct adherence of the metal deposit to the aluminum after suitable preliminary treatment, or by the use of an intermediate deposit, such as employed forexample in the zinc immersion process of plating an aluminum object with chromium. w

.The described stripping bath ismaintained at a 'temperature below'approximately F., a range of65 to 100 F. being a suitable operating temperature. The metal coated aluminum objectis made the anode in the bath and. subjected to a high current density. A suitable current density is between approximately four (4) and one hundred 'andjthirty amperes per square inch,

a current density range of between fifty (50) and seventy- 'five (75) amperes per square inch being preferred. A

potential of between four (4) and ten ('10) volts is used, it being understood that even higher voltages 'may be required'where the objectis irregular inshape and has regiven jcontour, the rate of removal of overlying metal is 'metalfdeposits are rapidly removed from the aluminum.

As .the metal overlying the aluminum is removed, the

current decreases until a zero reading appears on the ammeter to,indicate'that stripping has been completed, and clean, barealuminum hasbeen reached. Even though the now. stripped aluminum object is not immediately removed from the bath, my tests haveshown that the ,aluminum object 'is' not dissolved or loses any weight.

I am unable to explain with any degreeof certainty 'why' the action selectively ceases after the overlying inetaljor metals are removed from the aluminum, and does not remove aluminum. One possible explanation for the inertness of the underlying aluminum in my process, although aluminum ordinarily is soluble in dilute sulphuric acid, may he that a high oxygen overvoltage is formed at thefsurface of the aluminum object whichjfa'cts in somefmanner to prevent the aluminumtrom going into solution. The high over-voltage seemsto prevent any appreciable oxidation of the aluminum, oranodizingin depth; the voltage amperage relationshipjusedis outside the range which would'permit the aluminum to" be anodized to any. extent; Analysis has indicated that thereis no appreciable loss of aluminum to oxide as occurs when anodizing, 7

It is believed that the novel process of the present invention, as well as the advantages thereof will. beapparent from the' foregbing detailed description] "Whereas the underlying aluminum object is exposed, the current drops to zero and action ceases. The stripping solution is highly conductive, and the described conditions under, which the metal coated aluminum cally treated are such that'the. overlying metafcoating or coatings are very rapidly removed. It will also be apparentthatwhile theinvcntionhas been describedin a preferred form; changes m'ay be'made'witliout departing frornthespirit and scope'of the inventionyas sought to l, A process ofstrippinga metal coating from an underlying aluminumobje'ct, said processcomprising making the coated aluminumobiect 'an anode in a dilute aqueous solution consisting essentially of sulphuric acid v,haflving a concentration less than approximately 15% by volume, said solutionbeing maintained'jatfa' temperature' ,blOWi approximately 100" F., and, passing, current through said, solution, the current density beingin excess or approximately 4 amperes per square inch, the

oomple'te removal of coating metal being? indicated ,by a

by a zero reading of current.

3. A process ofstrlppmga metal coating from an underlying aluminum object; said process, comprising making the coated aluminum object an anode ina dilute aqueous solution"consistingfessentially of sulphuric acid having'a concentration lessithan approximately 115% by volume,said solution being maintained, at a temperature below approximately 100 F and passing current through said solution, the current density being approximately 4.

to l30 amperes per square inch, the complete removal of coating metal being indicated by'a zero reading of'current. 4.-A process of stripping .a metal coating from an underlying aluminum object; said process comprising.

making the coated aluminum object an anode in a dilute aqueous solution; consisting essentially of sulphuric, acid.

having a concentration'lof between approximately 2 to by volume, said. splution being maintained at a temperature below approximately 100 F; 1and.passing current through said solution at a potential in excess of approximately 4 volts, the'current density being in excess of approximately 4 amperes per square inch, the complete removal of coating metal being indicated by a zero reading of current. t I 5. Aprocess of, stripping agmetal coating froman underlying aluminum object, said 'process comprising making the coated aluminumobject an anode' in a dilute aqueous solution consistingessentially of sulphuric acid having a concentration of approximately 5% byvolume, said solution being maintained at a temperature between, approximately 65? and 100 F., passing current through said solution ata potential between approximately 4 and, 10 volts, the current densitylbeing between approximately 50 and 75 amperes persquare inch, the coinplete removal of coating metal being indicated by a zero reading of current I t u 6. A process of shipping a metal coating of the class consisting of chromium, nickel, copper and zinc from object is anodian underlying aluminum object, said process comprising making the coated aluminum object an anode in a dilute aqueous solution consisting essentially of sulphuric acid having a concentration less than approximately 15% by volume, said solution being maintained at a temperature below approximately 100 F., and passing current through said solution, the current density being in excess of approximately 4' amperes per square inch, the complete retempcraturebelow approximately 100 F., and passing current through said solution, the current density being in excess of approximately 4 amperes per square inch,

the complete removal of coating metal being indicated a zero reading of current.

8. A process of stripping a metal coating of the class consisting of chromium, nickel, copper and zinc from an underlying aluminum object,.said process comprising making the coated aluminumobject an anode in a dilute aqueous solution consisting essentially of sulphuric acid having a concentration less than approximately 15% by volume, said solution being maintained at a temperature below approximately 100? F., and passing current through said solution, the current density being approximately 4 to 130 amperes persquare inch, the complete removal 'of coating metal being indicated by a zero reading of current. v

'9. A process of stripping a metal coating of the class consisting of chromium, nickel, copper and zinc from an underlying aluminum object, said process comprising making the coated aluminum object an anode in a dilute aqueous solution consisting essentially of sulphuric acid having a concentration of between approximately 2 to 15% byvolume, ,said solution being maintained at a temperature below approximately 100 F., and passing current through said solution at a potential in excess of approximately 4 volts, the current density being in excess, of approximately 4 amperes per square inch, the

Lcompleteremoval of coating metal being indicated by a zero reading of current.

.10. A process of stripping a metal coating of the class consisting of chromium, nickel, copper and zinc from an'underlying aluminum object, said process comprising making the coated aluminum object an anode in a dilute aqueous solution consisting essentially of sulphuric acid having a concentration of approximately 5% by volume, .said solution being maintained at a temperature between approximately 65 and 100 F., and passing current .through said solution at a potential of between approximately 4 and 10 volts, the current density being approxi- I mately 50 to 75 amperes per square inch, the complete July 1937, p. 580; September 1937, Pp- 735-736.

removal of coating metal being indicated by a zero reading of current.

References Cited in the file of this patent Journal of the Electrodepositors Tech. 800., vol. 14, pp. 19-20, 1937-1938 and vol. 9, pp. 37-41, 19334934. Metal Cleaning and Finishing, May 1937, pp. 380-381; 

1. A PROCESS FOR STRIPPING A METAL COATING FROM AN UNDERLYING ALUMINUM OBJECT, SAID PROCESS COMPRISING MAKING THE COATED ALUMINUM OBJECT AN ANODE IN A DILUTE AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF SULPHURIC ACID HAVING A CONCENTRATION LESS THAN APPROXIMATELY 15% BY VOLUME, SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE BELOW APPROXIMATELY 100*F., AND PASSING CURRENT THROUGH SAID SOLUTION, THE CURRENT DENSITY BEING IN EXCESS OF APPROXIMATELY 4 AMPERES PER SQUARE INCH, THE COMPLETE REMOVAL OF COATING METAL BEING INDICATED BY A ZERO READING OF CURRENT. 